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New research at Harvard’s Department of Organismic and Evolutionary Biology sheds light on how infections can alter the social behavior of animals.

In an article published in Nature, Harvard OEB professor Yun Zhang and her colleagues studied the effects of the bacterium, Pseudomonas aeruginosa, on the small roundworm, Caenorhabditis elegans. The study specifically examined hermaphrodite roundworms, which are capable of self-reproduction.

Prior to infection, hermaphrodites exhibit a preference for self-reproduction; however, infection by the bacterium causes an elevated interest in the opposite sex. Zhang’s team discovered that infection alters the response of the host’s neurons to pheromones, which are secreted chemicals that trigger social responses in animals.

“We first found that a pair of chemical sensing neurons that usually do not sense pheromones start to respond to pheromones after worms were exposed to pathogenic bacteria,” Zhang said. “That was surprising because we did not know that this pair of neurons would respond to pheromones.”

The altered neuron causes a change in social behavior that could harm the host when interacting with other organisms of the same species. According to Zhang, pathogens may manipulate host organisms to aggregate more, thus increasing the pathogenic transmission rate.

Zhang’s team first encountered this project while working on a different research question. Taihong Wu and Minghai Ge, both postdoctoral fellows in the Zhang lab and co-first authors of the paper, were surprised when the pair of sensory neurons interacted with pheromones after the infection.

“It’s very important that after they found this surprise, they did not dismiss this finding,” Zhang said. “They actually stuck to this problem and tried to understand why and how this happened. That actually led us through this trajectory of the new project and opened a new direction of research.”

While this study is not directly related to human behavior post-pandemic, Zhang said it could be extrapolated to study the effects of pathogens on different organisms.

“There are lots of observations and studies that have examined social behavior changes in different animals,” Zhang said. “For social behaviors that are changed by interaction with pathogens, it is possible for people to look at whether in their organism of interest, pheromone receptors are also regulated and then whether this is the way that social behavior is changed in the host.”

“But what we do not yet understand a lot is how those social behaviors are changed at the level of the nervous system, at the level of the individual animals,” she added. “That’s where our findings contribute.”